Men are animals. On matters of eros, we accept this as a kind of psychological axiom. Men are tamed by society, yet the subduing isn't so complete as to hide their natural state, which announces itself in endless ways – through pornography, through promiscuity. Men are programmed by evolutionary forces to increase the odds that their genes will survive in perpetuity and hence they are compelled to spread their seed.

But why don't we say that women, too, are animals? Meredith Chivers, a psychologist trying to discover this, carried out research using a plethysmograph: a miniature bulb and light sensor placed inside the vagina. Semi-reclining, each of her female subjects watched an array of porn on an old, bulky computer monitor. The 2in-long tube of the plethysmograph beams light against the vaginal walls and reads the illumination that bounces back. In this way, it measures the blood flow to the vagina and finds out, at a primitive level, what turns women on.

As they enrolled in the study, Chivers' subjects identified themselves as straight or lesbian. They were shown images of sex between men and women, women and women, men and men, and a pair of bonobos (a species of ape). The subjects, straight and lesbian, were turned on right away by all of it, including the copulating apes. While they watched, they also held a keypad on which they rated their own feelings of arousal. So Chivers had physiological and self-reported scores. They hardly matched at all. Chivers' objective numbers, tracking what's technically called vaginal pulse amplitude, soared no matter who was on screen and regardless of what they were doing, to each other, to themselves. The keypad contradicted the plethysmograph entirely. The self-reports announced indifference to the bonobos. But that was only for starters. When the films were of women touching themselves or enmeshed with each other, the straight subjects said they were a lot less excited than their genitals declared. During the segments of gay male sex, the ratings of heterosexual women were even more muted.

Chivers put heterosexual and homosexual males through the same procedure. Strapped to their type of plethysmograph, they responded in predictable patterns she labelled "category specific". The straight men did swell slightly as they watched men masturbating and slightly more as they stared at men together, but this was dwarfed by their physiological arousal when the films featured women alone, women with men and, above all, women with women. Category specific applied still more to the gay males. Their readings jumped when men masturbated, rocketed when men had sex with men, and climbed, though less steeply, when the clips showed men with women; the plethysmograph rested close to dead when women owned the screen.

As for the bonobos, the genitals of both gay and straight men reacted to them the same way they did to the landscapes, to the pannings of mountains and plateaus. And with the men, the objective and subjective were in sync. Bodies and minds told the same story.

How to explain the conflict between what the women claimed and what their genitals said? Were the women either consciously diminishing or unconsciously blocking out the fact that a vast scope of things stoked them instantly toward lust?

The discord within Chivers' readings converged with the results of a study by Terri Fisher, a psychologist at Ohio State University, who asked 200 female and male undergraduates to complete a questionnaire dealing with masturbation and the use of porn. The subjects were split into groups and wrote their answers under three different conditions: either they were instructed to hand the finished questionnaire to a fellow student, who waited just beyond an open door and was able to watch the subjects work; or they were given explicit assurances that their answers would be kept anonymous; or they were hooked up to a fake polygraph machine, with bogus electrodes taped to their hands, forearms and necks.

The male replies were about the same under each of the three conditions, but for the females the circumstances were crucial. Many women in the first group said they'd never masturbated, never checked out anything X-rated. The women who were told they would have strict confidentiality answered yes a lot more. And those who thought they were wired to a lie detector replied almost identically to the men.

Fisher's research pointed to wilful denial. Yet, Chivers believed, something more subtle had to be at play. In journals, she found glimmers of evidence that women are less connected to the sensations of their bodies than men are, not just erotically but in other ways. Was this a product of genetic or societal codes? Were girls and women somehow taught to keep a psychic distance from their physical selves?

In a new experiment, Chivers played pornographic audio tapes for straight female subjects. She wanted to know, partly, whether spoken stories would have a different effect on the blood, on the mind. The scenes her subjects heard varied not only by whether they featured a man or a woman in the seductive role, but by whether the scenario involved someone unknown, known well as a friend, or known long as a lover.

Once again, the gap was dramatic: the subjects reported being much more turned on by the scenes starring males than by those with females; the plethysmograph contradicted them. But, this time, it was something else that interested Chivers. Genital blood throbbed when the tapes described X-rated episodes with female friends, but the throbbing for female strangers was twice as powerful. The male friends were deadening; with them, vaginal pulse almost flatlined. Male strangers stirred eight times more blood. Chivers' subjects maintained that the strangers aroused them least of all. The plethysmograph said the opposite: sex with strangers delivered a blood storm.

This didn't fit well with the societal assumption that female sexuality thrives on emotional connection, on established intimacy, on feelings of safety. Instead, the erotic might run best on something raw.

Sarah Blaffer Hrdy, a primatologist and anthropology professor, raised evolutionary reasons why this might be. Her ideas challenged evolutionary psychologists who insisted that women are the less libidinous sex, the sex more suited to monogamy. Hrdy had begun her career studying langur monkeys in India, whose males swoop in to kill newborns not their own. The same goes for the males in a number of other primate species. And female promiscuity among these types of monkeys and baboons evolved, Hrdy believed, partially as a shield: it masked paternity. If a male couldn't be sure which babies were his, he would be less prone to murder them.

Alongside this theory, she put forward an idea that revolved around orgasm. Female climax – in humans and, if it exists, in animals – has been viewed by many evolutionary psychologists as a biologically meaningless by-product with no effect on reproduction. Hrdy believed, however, that female orgasm could be thoroughly relevant among our ancestors. It was evolution's method of making sure that females are libertines, that they move efficiently from one round of sex to the next and frequently from one partner to the next, that they transfer the turn-on of one encounter to the stimulation of the next, building towards climax.

The possibility of multiple orgasms compounded libertine motives. The advantages female animals get from their pleasure-driven behaviour, Hrdy asserted, range from the safeguarding against infanticide in some primate species to, in all, gathering more varied sperm and so gaining better odds of genetic compatibility, of becoming pregnant, of bearing and raising healthy offspring.

In the end, recent science and women's stories left me with pointed lessons: that women's desire – its inherent range and innate power – is an underestimated and constrained force, even in our times. That, despite the notions our culture continues to imbue, this force is not, for the most part, sparked or sustained by emotional intimacy and safety. And that one of our most comforting assumptions – soothing perhaps above all to men, but clung to by both sexes – that female eros is much better made for monogamy than the male libido, is scarcely more than a fairytale.

• This is an edited extract from What Do Women Want? by Daniel Bergner, published next week by Canongate at £10.99. To order a copy for £8.79, including free mainland p&p, call 0330 333 6846, or go to theguardian.com/bookshop.